It has long been recognized that silicon nitride has particular utility with critically engineered applications, including, but not limited to engine components and cutting tools. In general, a single manufacturing facility fabricates completed silicon nitride powder and/or articles, requiring a great deal of investment, machinery, skilled labor, and facilities. It is anticipated that future facilities may necessitate the need of a starting material, i.e. one that has been preprocessed, so that end users may perform the final processing steps in order to achieve the manufactured item that they desire.
Reaction bonded silicon nitride is commonly prepared by reacting and nitriding the silicon (either as a powder or as a formed article) with nitrogen by exposing the silicon to a nitrogen-containing atmosphere at temperatures of 1100.degree. C. to about 1420.degree. C. for times sufficient to produce the silicon nitride. It is not uncommon for the nitriding time in prior art methods to be 100-200 hours. It is normal for a small amount of nitriding aid (e.g., iron oxide or nickel oxide) to be initially mixed with the silicon powder to enhance the nitridation of the silicon during the nitriding step.
As described in U.S. Pat. No. 4,943,401 to the present inventors, also assigned to Eaton, which is also incorporated herein by reference, a method is disclosed for producing a silicon nitride material by comminuting silicon powder with water. Further disclosed are a nitriding agent, a sintering aid, and the process for producing the silicon nitride from start to finish, through sintering.
It has been recognized by the present inventors that it may be advantageous to prepare a starting material for end users to facilitate certain processing steps at their plants. Therefore, it would be helpful if the end users would have a source of good starting material to which they would be able to add any nitriding agents, densification aids, or other desired additives to aid in their processing, which are relatively quick steps in relation to the relatively long step of preparing the nitridable silicon-containing starting material. As discussed above, prior art methods for producing silicon nitride take a very long time. The comminuting step as disclosed in U.S. Pat. No. 4,943,401 greatly reduces the processing time. It is envisioned that a bulk processing plant could produce the initially reacted silicon-containing material which is necessary for end processing, while selling various formulas and additives to be included with the comminuted slurry, or a resulting dry mass for particular applications.
Potential end processors would be able to purchase the nitridable silicon-containing starting material, along with various additives, including nitriding aids, densification aids, and other processing aids, such as lubricants, binders, and other organic materials, which would be able to produce a desired material tailor made for their application. The end user would not need to purchase all of the reaction equipment which is necessary for producing the nitridable silicon-containing material. Rather, they would be required to purchase a small mixer and could mix in any combination of additives which they may desire. Thereafter, they could form articles from the silicon-containing material and nitride them in their desired shape. Thereafter, as is well known in the art, they could sinter the material to form a densified silicon nitride material which is relatively high in strength.
It is widely known that if common sintering or densification aids for silicon nitride (e.g., magnesium oxide, yttrium oxide, aluminum oxide, rare earth oxides, etc.) are mixed in initially with the silicon powder, the reaction bonded silicon nitride article, provided it has a high alpha phase content, can be further heated immediately after nitriding to the higher sintering temperature and will thereby be sintered to increase its density and have improved mechanical properties as compared to the original reaction bonded silicon nitride article.
The article can be hot pressed, hot isostatically pressed, pressure assisted sintered, or pressureless sintered, and may be covered with a cover powder during the sintering process to prevent any degradation. Boron nitride or silicon nitride, or mixtures thereof are commonly employed as a cover powder over the article during sintering to minimize the decomposition of the reaction bonded silicon nitride article. During sintering, the alpha phase of the silicon nitride material is converted to the beta phase of silicon nitride.
Therefore, high levels of alpha phase silicon nitride need to be present in the pre-sintered reaction bonded silicon nitride to obtain the desired sintering response. In the past, it has been found that some methods produce a high beta phase material too early in the process to be useful. A sinterable silicon-containing starting material is generally in the form of alpha phase materials, if processed properly, although prior art methods have produced an undesirably high level of beta phase material. The following paragraphs describe prior art methods and materials for producing silicon nitride articles.
U.S. Pat. No. 3,206,318 to Yamauchi et al. teaches a method of nitriding metallic silicon which lowers the ill effects of the oxidation of silicon nitride, in which the nitriding catalyst is (a) at least one primary substance selected from the group consisting of metallic vanadium, the inorganic compounds thereof, and mixtures thereof; or (b) that comprising (a) in which has been incorporated at least one secondary substance, selected from the group consisting of metallic cobalt, manganese, chromium, copper, nickel, iron, barium, and calcium and the inorganic compounds thereof. Yamauchi, et al. also teach a refractory article in which granular refractory material, such as alumina, is bonded with silicon nitride. The patent furthermore teaches that the oxides of the metals, Cu, Co, Ni, Cr, Mn and V, may likewise be used and that the quantity of these oxides is suitably 0.1-2 moles in terms of the metallic element to 100 moles of the silicon.
U.S. Pat. No. 4,235,857, METHOD OF NITRIDING SILICON, to Mangels teaches that silicon can be nitrided using a demand nitriding cycle over the temperature range of 900.degree. C. to 1420.degree. C. in an atmosphere consisting of a mixture of nitrogen, hydrogen and helium. The chemical composition of the nitriding gas is constantly changing during the nitridation of the silicon article, with the chemical activity of the nitrogen decreasing (partial pressure of nitrogen in the furnace decreases) as the temperature increases. The examples cited by Mangles have nitriding times of from 130 to 175 hours.
U.S. Pat. No. 4,351,787 to Martinengo et al. teaches that sintered silicon nitride articles can be prepared by forming a silicon powder mixture containing one or more sintering additives into a compact, the additives being present in the powder in an amount such as to ensure an additive content of from 0.5 to 20 wt. % in the silicon nitride compact; heating the compact under a nitrogen gas blanket at a temperature not exceeding 1500.degree. C. to convert the silicon into reaction bonded silicon nitride; and sintering the reaction bonded silicon nitride compact by heating in a nitrogen gas atmosphere at a temperature of at least 1500.degree. C. Furthermore, it is taught that the silicon powder size is from 0.1 to 44 microns in size and of high purity or containing only very small amounts of nitriding catalysts. The Martinengo et al. patent teaches that any conventional sintering additive may be used. Best results are said to be achieved by using MgO, and especially in combination with Y.sub.2 O.sub.3. Other preferred additives mentioned in the patent are MgO, Y.sub.2 O.sub.3, CeO.sub.2, ZrO.sub.2, BeO, Mg.sub.3 N.sub.2, and AlN. Other examples of additives are given as Mg.sub.2 Si, MgAl.sub.2 O.sub.4, and rare earth additions such as La.sub.2 O.sub.3. Also iron can be used with advantage, usually in mixture with conventional additives such as MgO, Y.sub.2 O.sub.3, and CeO.sub.2.
It is, therefore, a primary object of the present invention to provide an improved process for making a body of nitridable silicon-containing material which can later be processed as desired which is more commercially viable than prior art methods, uses materials which are readily available, and is processable in substantially less time than typical prior art methods.